Turbine buckets are subjected to a gas force that provides torque to the rotor. Relatively small variations in these gas forces such as, for example, upstream nozzle throat area induced harmonic distortion of the hot gas airflow, can cause bucket vibration. Coincidence of resonance between these nozzle-induced periodic gas forces and bucket natural frequencies must be avoided at full operating speed; however, resonance cannot be avoided at all speeds, particularly during starting and shutdown. Effective vibration control is required, therefore, to produce reliable turbine designs.
The development of turbine stages that are vibration free requires sophisticated interaction between the aerodynamic design and testing disciplines. For free-standing buckets, the calculation of frequencies is relatively routine; however, the amplitude of vibration response of buckets to aerodynamic stimulus is not easily determined without extensive test correlations. When the complexities of variable boundary conditions at platform and tip shroud are introduced into the assembly, analytical predictions become even more uncertain. Extensive test experience is required, therefore, to produce a reliable design.
Currently, time consuming and error prone nozzle throat area hand measurements are taken after the nozzles are assembled into the structural support casing. These measurements are then hand typed into a computer that is programmed to calculate the relative harmonic contents of the per revolution excitation forces by fourier series analysis techniques. If the preestablished criterion are not met, the nozzles are removed and other sequences are tried until the criterion are met. Nozzle throat area checks are made at every turbine stage to ensure that the amount of periodic excitation forces caused by variations in the circumferential spacing and manufactured shape of the nozzle segments does not exceed maximum values. Also, periodic placement of upstream structural components induces circumferential varying gas forces. The buckets, which follow the nozzles in the gas path, are subject to these periodic gas force pulses as they pass by the nozzles, which could excite one of their natural frequencies of vibration, causing ultimate catastrophic failure of the unit.